Fabric woven with flat glass fibers and production method

ABSTRACT

Method of production of glass fabric that does not exhibit surface hairiness, that is sufficiently close-woven, with which printed circuits with a high degree or rigidity can be realized, as well at supply of a glass fabric, prepreg and print circuits. Glass fabric ( 40 ) woven from warp yarn ( 20 ) and weft yarn ( 30 ) containing multiple glass filaments; in relation to the warp yarn ( 20 ) and the weft yarn ( 30 ), at least one of the two has not undergone twisting and the profile of its glass filament has been flattened. Since between warp yarn ( 20 ) and weft yarn ( 30 ), at least one of the two has not undergone twisting, it is possible to limit the surface hairiness and at the same time to obtain that the fabrics arm separated and uniformly distributed during the weaving operation also without performing a specific treatment of uniform fibers redistribution. Moreover, since between warp yarn ( 20 ) and weft yarn ( 30 ), the filament F of at least one of the two has a flattened profile, the spacing between the filaments F is closer, the degree of distribution of the fibers of the glass fabric is higher and the rigidly of the printed circuit laminate is greater.

FIELD OF THE INVENTION

The present invention relates to a woven glass fabric and to productionmethods of a woven glass fabric obtained by weaving warp yarns and weftyarns.

BACKGROUND OF THE INVENTION

Thanks to the rapid developments that have occurred lately in theelectronic industry, the printed circuits, that constitute the supporton which integrated circuits and semiconductors are mounted, undergocontinuous improvements. At the base of the printed circuit, thereinforcement material used for electrical insulation is impregnatedwith a resin and then left to harden. As reinforcement material, a wovenglass fabric is generally chosen with a plain weave construction formedby a weft and a warp made of continuous glass fibers.

With the improvements in the impregnation capabilities of the resins, ithas become necessary to implement research efforts in the direction of agreater homogeneity of distribution of the fibers in the woven glassfabric used for the printed circuits. Hence, the best distribution ofthe spaces between the yarns of the woven fabric is obtained utilizingmethods aimed at the uniform redistribution of fibers, such as thetreatment with sprays of jets of fluid, or the vibration of a fluid, ormoreover with direct pressure. In other words, the goal is to fill thespaces and interstices resulting from the weaving process of the wovenfabric to render uniform the woven fabric made of weft and warp yarns,since the yarns may tend to separate during the process.

Up to today, in the known woven glass fabrics the following problems arefound: with the introduction of terminals of reduced dimensions, moremobile, and the orientation towards high performance materials, anincreasing need for thin glass fabrics has emerged, but in which theinterstices are however accurately filled. In the case of thin glassfiber fabrics, executing the uniform redistribution of the fibers withjets of fluid or the vibration of a fluid, the fibers tend to separateand break, and moreover the mesh of the fabric is distorted. Trying tolimit said problem of separation of the yarns, it is not possible toreach the goal of uniform redistribution of fibers, and thereforefilling the interstices becomes problematic.

In the attempt to obtain thin printed circuits, their rigidity isdiminished. Therefore, there is the need to obtain a thinner glass fiberfabric, and at the same time to increase the rigidity of the printedcircuits that use woven glass fibers fabric.

The present invention has the goal to provide a production method for awoven glass fiber fabric, in which the phenomenon of surface hairinessdoes not arise, in which the interstices of the fabric can be filled ina satisfactory way, and with which it is possible to obtain printedcircuits with a high rigidity; moreover, the goal of the invention is toprovide said woven glass fiber fabric, as well as prepreg and printedcircuits.

DESCRIPTION OF THE INVENTION

In order to obtain the goals listed above, the invention provides aproduction method for a woven glass fabric made up of warp yarns andweft yarns composed of multiple filaments; said production method beingcharacterized:

-   -   By a phase in which the fused glass is spun, conferring to said        filaments a flattened profile;    -   By a phase in which said glass filaments are assembled so as to        form a yarn of continuous glass fibers;    -   By a phase in which said yarn of continuous glass fibers is        wound in a way to form a forming cake;    -   By a phase in which said yarn of continuous glass fibers is used        to weave the glass fabric object of the invention, without        having to resort to the process of rewinding of said continuous        glass fibers yarn previously wound on said forming cake.

Up to now, in the production methods used to make woven glass fabrics,the glass filaments, after having been assembled with an appropriatesize so as to form a yarn of continuous fibers, are wound onto a formingcake using a winder. Subsequently, the continuous glass fiber yarn,wound on a forming cake, is rewound on a bobbin and finally used for theproduction of glass fabrics. Moreover, a process of twisting thecontinuous glass fibers is generally coupled to the process of rewindingthrough a winder onto a bobbin.

On the contrary, in the glass fabric production method of the currentinvention, after the continuous glass fibers have been wound onto aforming cake, these are used in the weaving operation of the glass fiberfabric without having to go through the rewinding process beforehand.Therefore, being able to avoid the rewinding process, in which case thecontinuous glass fibers are not further twisted, it is then possible tolimit the phenomenon of surface hairiness. In the case in which theprofile of the glass filament is flattened, since its outer surface ismore extensive than a filament with a round profile, the surfacehairiness due to the friction between the glass filaments themselvestends to take place more easily. Consequently, the possibility to avoidthe rewinding process allows a remarkable limitation of the phenomenonof surface hairiness. Moreover, since the continuous glass fibers arenot twisted, the packing strength of the filaments is weaker, andtherefore, even without proceeding to an operation of uniformredistribution of the fibers, these separate naturally, resulting in awoven fabric with a high degree of filling of the spaces. Moreover,since the glass filament has a flattened profile, the thickness of thepart in which filaments overlap one another is thinner with respect towhat happens in the case of filaments with a round profile;consequently, the dimensions of the interstices between filaments insideof the woven fabric are reduced. Therefore, if we compare a glass fiberfabric in which a round profile filament has been used, to another inwhich a flattened profile filament has been used, assuming that theyhave the same thickness, the second will provide a greater degree ofdistribution of the fibers. Consequently, the printed circuits made withglass fiber fabrics obtained according to the method of the presentinvention have greater rigidity.

Moreover, in relation to the manufacturing method object of the presentinvention, when a yarn formed by glass fibers with a flattened profileis used only in the weft or only in the warp, it will be characterizedby continuous fibers that have not undergone a rewinding process.

In the case in which continuous glass fibers of a flattened profile,that do not need a rewinding process, are only used in the weft or onlyin the warp, the glass fiber fabric does not exhibit surface hairinessproblems and it appears as a close-woven fabric, moreover printedcircuits with a high degree of rigidity can be obtained.

Other methods of production of the glass fiber fabric related to thepresent invention comprise a method of production of a glass fiberfabric in which the weft and the warp consist of a glass yarn withmultiple filaments. Said production method is formed by a phase in whichthe fused glass is spun conferring to said filaments a flattenedprofile; a phase in which said glass filaments become assembled as toform a continuous glass fiber yarn; a phase in which the continuousglass fiber yarn is wound as to form a forming cake; a phase in whichsaid yarn of continuous glass fibers is woven without having to resortto the process of twisting of the yarn of continuous glass fibers thatwas previously wound onto a forming cake.

According to the method of production of the glass fiber fabricpertaining to the present invention, the weaving process of the glassfiber fabric is performed without passing through the process oftwisting of continuous glass fibers after their winding onto a formingcake. Avoiding in this way the twisting operation of the continuousglass fibers, it is possible to limit the surface hairiness due to thetwisting of the continuous glass fibers. In the case in which theprofile of the glass filament is flattened, since its surface is moreextensive than the one of a filament with a round profile, the surfacehairiness due to friction between the glass filaments themselves tendsto take place more easily, consequently the possibility to avoid therewinding process helps remarkably to limit the phenomenon of surfacehairiness. Moreover, since the continuous glass fibers are not twisted,the compaction force of the filaments is weaker, therefore even withoutproceeding to an operation to uniformly redistribute the fibers, thesenaturally separate during the weaving process, allowing to obtain a gooddegree of filling of the interstitial spaces. Moreover, since the glassfilament has a flattened profile, the thickness of the side wherefilaments overlap one another, on the shorter side of the profile, isthinner compared to the case of a filament with a round profile;consequently, the dimensions of the interstices between filaments insideof the fabric are reduced. Therefore, if we compare a glass fiber fabricin which a filament with a round profile has been used to one in which afilament with a flattened profile has been used, assuming that they havethe same thickness, the latter will possess a greater degree ofdistribution of the fibers. Consequently, the printed circuitsmanufactured with a glass fiber fabric obtained according to the methodof the present invention possess greater rigidity.

The method of production of the glass fiber fabric pertaining to thepresent invention is considered appropriate in case the weft or the warphave glass filaments with a flattened cross-section, and in case thecontinuous glass fibers have not undergone a twisting process.

In the case in which continuous glass fibers with a flattened profilethat do not need a rewinding process are used only in the weft yarn oronly in the warp yarn, the glass fiber fabric does not exhibit problemsof surface hairiness, and a satisfactory degree of filling of theinterstitial spaces is obtained, moreover printed circuits are obtainedwith a high degree of rigidity.

The method of production of the prepreg of the present invention ischaracterized by the following procedure: a glass fiber fabric obtainedfollowing any of the methods of production presented above, isimpregnated in a thermosetting resin and is brought to a semi-rigidstate.

According to the method of production of the prepreg pertaining to thepresent invention, using the methods of production presented above;surface hairiness is not found in the continuous glass fibers dipped inthe thermosetting resin, the distribution of the fibers is nearlycompletely uniform, moreover the printed circuits using such prepregpossess comparatively greater rigidity.

The glass fiber fabric of the present invention is a glass fabriccomposed by a weft and a warp each containing multiple filaments, inwhich the yarns of at least one of the two components, weft or warp, donot undergo a twisting process, and in which the profile of the filamentis given a flattened shape.

In the case of the fabric of the present invention, since the yarn of atleast one of the two components, weft or warp, has not undergone atwisting process, allowing in such a way a softer compaction of thefibers, the uniform distribution of the fibers happens naturally, alsowithout proceeding to specific redistribution operation, and acloser-woven fabric is obtained. Moreover, since the yarn of at leastone of the two components, weft or warp, has not undergone a twistingprocess, the problem of the surface hairiness due to the twistingprocess is limited. In the case in which the profile of the glassfilament is flattened, being its surface more extensive than that of around filament, the surface hairiness due to the friction between glassfilaments happens more easily. The fact that the surface hairiness islimited is undoubtedly an advantage. Moreover, since the glass filamenthas a flattened profile, the thickness of the side in which filamentsoverlap one another, on the shorter side of the profile, is thinner thanwhat happens in the case of a filament with a round profile;consequently the dimensions of the interstices between filaments insidethe fabric are reduced. Therefore, if we compare a glass fiber fabric,in which a filament with a round profile has been used to another inwhich a filament with a flattened profile has been used, assuming thatthey have the same thickness, the latter will have a greater degree ofdistribution of the fibers. This is the reason why a printed circuitproduced with the glass fiber fabric described in the present inventionexhibits a comparatively greater rigidity.

In the case of the glass fiber fabric of the present invention it isconsidered adequate that the warp yarn has been twisted and has asummarily round profile, whereas the weft yarn has not been twisted andhas a flattened profile.

Since the yarn of at least one of the two components of the fabric, weftor warp, has not undergone a twisting process, the compaction of thefibers is softer, and even without proceeding to a specific fiberredistribution operation, the uniform distribution of fibers happensnaturally, allowing to obtain a very close-woven fabric. Moreover, ifonly the weft yarn is not twisted, the surface hairiness of weft yarndue to twisting can be limited.

The prepreg of the present invention is obtained from said woven glassfiber fabric, which is impregnated by a thermosetting resin. In theprepreg of the present invention, since said fabric is used, the weftyarn inside the thermosetting resin does not exhibit surface hairinessand a nearly regular distribution of the fibers is obtained. It followsthat the rigidity of the printed circuits produced with such prepregs iscomparatively greater. Such results are even more evident if the weftyarn has not been twisted and if a prepreg is employed in which afilament with flattened section has been used.

The printed circuits of the present invention include the abovementioned glass fiber fabrics as base material. In the case of theprinted circuits object of the present invention, since said fabrics areused, the weft yarn does not exhibit surface hairiness, the distributionof the fibers is nearly uniform and the rigidity is comparativelyhigher.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail, in an exemplifying way,with reference to the attached Figures, in which it must be noted thatthe same numbers correspond to the same elements.

FIG. 1 is a schematic diagram of the structure of the glass fiberproduction system used to produce continuous glass fibers;

FIG. 2 represents the cross-sectional view, as seen from the bottom, ofthe bushing employed in the glass fiber production system of FIG. 1;

FIG. 3 illustrates an automatic loom;

FIG. 4 represents a practical application of the glass fabric object ofthe present invention;

FIG. 5 represents section VV of the glass fabric of FIG. 4;

FIG. 6 is a cross-sectional view of a practical application of theprepreg of the present invention;

FIG. 7 is a cross-sectional view of a practical application of theprinted circuit of the present invention;

FIG. 8 represents the short and the long side of the filament;

FIG. 9 illustrates the rate of distribution of the fibers of the warpand weft yarn.

Referring to the Figures from 1 to 3 the method of production of theglass fabric of the present invention will be explained. First of all,we will refer to FIGS. 1 and 2 in order to explain the production phasesstarting from the fused glass until the formation of continuous glassfibers. The first Figure represents in abbreviated form the system 1 ofproduction of glass fibers in order to obtain continuous glass fibers.In the system 1 of the production of glass fibers the bushing 10 ispositioned at the base, while a crucible 2 is placed in which the fusedglass is fed; the fused glass, that comes out from the holes of bushing10, after having been cooled, becomes the filament F.

FIG. 2 is a diagonal view from the bottom side. In the bushing 10, aflattened shape has been given to holes 12. Moreover, two holes 12constitute a couple, and between every couple of holes 12 there is arectangular-shaped socket 14 for the separation of filaments. The fusedglass that comes out from each hole 12 is moulded according to the shapeof the holes 12, it exhibits a flattened profile and is cooled as itcontacts the air present in the unit of separation 14. The fused glasswith a flattened profile increases its own viscosity and it solidifieseven before decreasing the degree of flattening due to the surfacetension, creating in this way the glass filament of flattened profile F.

The production process of glass fibers will be illustrated withreference to FIG. 1. Under the bushing 10 a roller 4 is placed to applythe binder. Thanks to said roller 4 used for the binder application,this binder is applied on the glass filament F, which, under the effectof the joining cylinder 6 is assembled and formed in yarn; in this waythe continuous glass fiber S is obtained. In a continuous glass fiber S,for example, are contained between 50 and 400 glass filaments F.Subsequently, the yarn is wound in a cylindrical or conical shape withthe winder 8, and takes the form of a forming cake 9. But it must benoted that at the state of forming cake 9, the continuous glass fiberhas not undergone twisting.

Below we will be examining the automatic loom 41 of FIG. 3 and explainthe weaving process of the glass fabric. First of all, the forming cakeof the continuous glass fibers 9 obtained as explained above, ispositioned “as is” in the weft feeder 42. This means that, as far as theweft yarn, the rewinding operation onto a bobbin is not performed as ithas been performed up to today. Since the rewinding operation is notperformed starting from the forming cake 9, therefore the continuousglass fibers are not twisted. As far as the warp yarn, in the presentapplication the profile of the filament is roughly round and thecontinuous glass fibers have been rewound from forming cake to bobbin.However as far as warp yarn (not the weft yarn), a filament with aflattened profile can be used that has not been rewound from the formingcake.

After having loaded on the automatic loom 41 the weft yarn and warp yarnmade of continuous fibers, the loom 41 is started and the multiple yarnsof warp yarn 20 are inserted in the same direction with a uniformspacing, while the weft yarn 30 is inserted in such a way to cross thewarp yarn at a 90 degree angle. In this way glass fiber fabric isobtained with a plain weave construction.

FIG. 4 is a depiction of the glass fiber fabric 40 thus obtained, whileFIG. 5 is the relative cross-section in the VV direction of FIG. 4. Itis noted that FIG. 5 is a cross-section that follows the line that markshalf of the width of warp yarn 20 (right-left direction in FIG. 4). Asit is illustrated in each of the Figures, warp yarn 20 and weft yarn 30cross each other overpassing one another, alternatively.

As described above, as far as the fabrication of the glass fiber fabric40, the continuous glass fiber S, after having been wound in acylindrical shape onto forming cake 9, is then woven in the glass fabric40 without undergoing any rewinding operation onto a bobbin, that iswithout imposing to the continuous glass fiber S a further twistingprocess. If the rewinding process of the continuous fibers is avoided(twisting process), and continuous glass fibers S (weft yarn 30) do notundergo twisting, it is possible to limit the surface hairiness.Moreover, since the weft yarn has not undergone twisting, its degree ofcompaction is weaker; that allows to obtain a uniform distribution ofthe fibers in a natural way with the force exercised by the warp yarn20, without having to perform a specific separate operation with thegoal of obtaining a uniform distribution of the fibers. Moreover, evenif the filament is not distributed in an uniform way, when the force ofthe warp yarn is exercised, since the secondary moment for the glassfiber filament with a flattened profile is greater and since also therotation radius is greater, the filament tends to position itself in thedirection of the short side, therefore resulting in a thin yarn with awide surface.

Moreover, also in the case in which any treatment for the uniformredistribution of fibers is applied to the glass fibers fabric 40, suchas a spray of jets of liquid or the vibration of a liquid, inasmuch asthe continuous glass fiber S has not undergone any twisting operationand therefore its degree of compaction is weak, a satisfactory uniformredistribution of the fibers is obtained even if the energy applied tothe glass fiber fabric 40 at the moment of the operation of fibersredistribution is smaller. The result is a close-woven glass fiberfabric 40 and at the same time a limited surface hairiness. As examplesof energy used during the operation of uniform redistribution of fibers,we can mention the speed or the power of the jet of liquid in the caseof a spray operation, or the frequency of vibration in case of atreatment with vibration of a liquid. It is possible to set the speed ofsuch treatments so as to vary the degree of distribution of the fibers.

Being the glass filament F that constitutes the weft yarn 30 offlattened profile, at the moment of the weaving process the variousglass filaments F are inserted with their length positioned horizontallyas evidenced in FIG. 5. Moreover, since the profile of the filament F isflattened, the interstices between the contiguous filaments F are ofsmaller dimensions, that is, the filaments F are closer betweenthemselves. Moreover, the thickness of the glass fabric 40 is thinner inthe direction in which the short sides of the section of filaments F areoverlapped (top-bottom direction in FIG. 5).

With reference to FIGS. 6 and 7, explanations are now given about theprepreg and the printed circuits in which the glass fiber fabric 40 isused. FIG. 6 is an oblique view of prepreg 50 manufactured with theglass fiber fabric 40 of the present practical application, while FIG. 7represents printed circuit 60 manufactured with prepreg 50 as basematerial. For prepreg 50, after having impregnated the glass fabric 40with thermosetting resin that has a resin-matrix function, a dryingtreatment is applied after which prepreg 50 will be in a semi-rigidstate. As far as the resin-matrix, practically epoxy resins,non-saturated polyester resins and polyimide resins and other similarresins are used.

Printed circuit (PWB: Printed Wiring Board) 60 formed with prepreg 50 asbase material can be obtained as described below. At first, severalprepregs are overlapped on each other and, using pressure and heat, amultilayer laminate is obtained. To the laminate that has undergone apressure and temperature treatment is attributed 50 a. On both sides (oralso on a single side) of the multilayer laminate a copper sheet isapplied and a copper clad laminate is obtained (CCL: Copper CladLaminate). Following a method such as the so called “subtractive”method, line 54 is shaped, which constitutes the printed circuit. Inthis way the printed circuit 60 is completed as it is represented inFIG. 7. However, the printed circuits of the present invention are notlimited to what is depicted in FIG. 7, as a large number of variationsare possible. As an example, instead of limiting the printed circuit tothe external surface, it can be realized also in the inner layers, thusobtaining multilayer printed circuits (ML-PWB: Multilayer Printed WiringBoard).

Said prepreg 50 comprises the glass fabric 40 impregnated in thethermosetting resin; said glass fabric 40 exhibits weft yarn of uniformdistribution and without surface hairiness. This is the reason why theprinted circuit 60 made with said prepreg 50 is of homogenous rigidityin all points. Moreover, as already explained previously, since theprofile of the filament F is flattened, the position of the filaments Finside the prepreg 50 is very close; this confers greater rigidity toprinted circuit 60, which takes advantage of the prepreg 50 for itsfabrication. Based on such premises, it is possible to obtain a thinprinted circuit 60, but with a high degree of rigidity.

In the present practical application, the continuous glass fiber Sobtained from the glass fiber filament of flattened profile F that hasnot undergone the rewinding twisting operation, is not used in the warpyarn and in the weft yarn at the same time, but only in the weft yarn.However, also in such circumstances it is possible to limit the surfacehairiness at the moment of the uniform redistribution of the fibers andobtain a close-woven fabric in a satisfactory way, and also a printedcircuit 60 with high rigidity. However, differently from the presentpractical application, in which the continuous glass fiber S that hasbeen obtained from the filament F and that has not undergone therewinding and twisting process is used only in the weft 20, it is anywaypossible to use said continuous glass fiber S only in warp 30.Obviously, it is also possible to use the continuous glass fiber S thathas been obtained from the filament F that has not undergone therewinding and twisting process both for warp 20 and for weft 30.

Examples of Practical Applications

Below, based on practical applications, the results of the presentinvention will be examined more concretely.

As evidenced in Table 1, in the example of practical application 1, thewarp yarn ECE225 is formed by a round glass filament of 7 μm diameter,and the weft yarn is formed by a glass, filament with a flattenedprofile whose short side is of 4.5 μm and long side of 18 μm. The longand short sides express the length depicted in FIG. 8. Since the warpyarn has been rewound on bobbin, after having been spun and wound on aforming cake, it possesses a twist of 1.0 turns/25 mm. The weft yarninstead, not being rewound, has not undergone any twisting operation. Onthe glass fabric formed by such weft and warp, no operation has beenperformed for the uniform redistribution of the fibers. TABLE 1 Exampleof practical Comparative Comparative application 1 example 1 example 2Continuous Filament Warp 7 7 7 glass Diameter Weft Short side 4.5 Shortside 4.5 9 fibers (μm) Long Side 18 Long Side 18 yarns (flattened(flattened profile) profile) Yarn Count Warp 22.5 22.5 22.5 Weft 33.733.7 33.7 Torsion Warp 0.7 Z 0.7 Z 0.7 Z (turns/25 mm) Weft 0 Z (no 1.0Z 0 Z (no rewinding) rewinding) Density of the fabric Warp 40 (yarns/25mm) Weft 39 Air permeability (cm³/cm²/s) 41.1 119.6 132.9 Unit weight(g/cm²) 88.1 Thickness (millimeter) 0.082 0.09 0.093 Fabric degree ofimpregnation (%) 41.3 37.7 36.4

Having carried out the measurement of air permeability of this glassfabric in a conforming apparatus (JIS R 3420), a relatively low value of41.1 cm³/cm²/s is resulted. That means that even if no operation ofuniform redistribution of the fibers has been performed, the glassfabric of practical application 1 exhibits a low air permeability and aclosed weave. That is because the weft yarn with a flattened profile hasnot been twisted, because its degree of compaction is low and becausethe uniform distribution of the fibers has happened in a natural way inthe weaving phase. It is noted however that the glass fabric, afterhaving been impregnated in the resin, reaches a degree of impregnationof the glass fibers of 41.3% inside of the hardened multilayer.Moreover, by examination of the surface of the glass fabric, no surfacehairiness of the weft yarn is found. This is due to the fact that it waspossible to avoid the twisting operation.

After having measured the degree of air permeability, the treatment ofuniform redistribution of the fibers of the glass fabric of thepractical application 1 with a liquid (water) was performed. The glassfabric that before the operation of uniform redistribution of the fibershad a degree of distribution of the fibers of the warp yarn of 49.7%,and a degree of distribution of the fibers of the weft yarn of 95.9%,with an air permeability of 41.1 cm³/cm²/s, has exhibited a degree ofdistribution of the fibers of the warp yarn of 81.6% and a degree ofdistribution of the fibers of the weft yarn of 97.6%, with an airpermeability of 13.2 cm³/cm²/s. By examination of the surface of theglass fabric after the uniform redistribution of the fibers, almost nosign of surface hairiness has been found. Moreover, these results allowto underline that, in the case of the glass fabric of the currentexample of practical application, the uniform redistribution of thefibers is obtained more easily for the warp yarn than for the weft yarn.The degree of distribution of the fibers of the warp yarn and of theweft yarn, with reference to FIG. 9, has been calculated in thefollowing way:

-   -   degree of distribution of the fibers of the warp yarn=A1/A×100        (%) degree of distribution of the fibers of the weft        yarn=B1/B×100 (%)

-   A=warp yarn spacing

-   A1=width of the warp yarn

-   B=weft yarn spacing

-   B1=width of the weft yarn.

In the glass fabric of comparative example 1, for warp yarn and weftyarn the same yarn of the example of practical application 1 has beenused. The part in which the comparative example diverges from theexample of practical application 1 is the following: before weaving, theweft yarn is rewound on a bobbin with a torsion of 1.0 turns/25 mm. Asin the example of practical application 1, the measurement of airpermeability of this glass fabric has been carried out in a conformingapparatus (JIS R 3420) and the result is 119.6 cm³/cm²/s. This resultshows that the glass fabric of comparative example 1, with respect tothe one of the example of practical application 1, is less close-woven.This is due to the fact that the weft yarn with a flattened profile hasbeen twisted, the degree of compaction of the fibers is high, andtherefore the weft yarn does not widen easily during weaving. Moreover,since in comparative example 1 the rewinding of the weft yarn has beenperformed, we observe a substantial surface hairiness of the weft yarnof the glass fabric.

Differently from the example of practical application 1, for the glassfabric of comparative example 2 a glass filament of round profile(diameter 9 μm) has been used. However, as in the example of practicalapplication 1, the weft yarn has not been rewound from the forming caketo the bobbin and no twisting operation has been performed. As in theexample of practical application 1, the measurement of air permeabilityof said glass fabric has been performed in a conforming apparatus (JIS R3420) and the result is. 132.9 cm³/cm²/s. This result shows that theglass fabric of comparative example 2, with respect to the one of theexample of practical application 1, is less close-woven. In this way itis evident that simply not performing the twisting of the weft or warpyarn is not enough to increase the uniform distribution of the fibers.The improvement of the degree of uniform distribution of the fibers isconnected to the use of a filament with a flattened profile for thecontinuous glass fibers, and to the lack of twisting.

1. Method for the preparation of a glass fabric comprising weft yarnsand warp yarns containing multiple filaments of glass, which methodcomprises: a phase in which the fused glass is spun to form saidfilaments with a flattened section; a phase in which said glassfilaments are assembled so as to form a continuous glass fiber yarn; aphase in which said yarn of continuous glass fibers is wound so as toform a forming cake; a phase in which said yarn of continuous glassfibers is used to weave said glass fabric object of the inventionwithout having to resort to the process of rewinding said glass yarn ofcontinuous fibers previously wound on said forming cake.
 2. Method toproduce a glass fabric woven according to claim 1, in which said fabricis comprised of said warp and said weft, whose glass filaments arecharacterized by said flattened section, and which fabric is moreovercharacterized by being formed by said continuous glass fibers that donot need a rewinding process.
 3. Method for the preparation of a glassfabric in which the weft and the warp consist of a glass yarn ofmultiple filaments, comprising the following phases: spinning the fusedglass conferring to said filaments a flattened section; assembling saidglass filaments in a way as to form a continuous glass fiber yarn;winding said yarn of continuous glass fibers as to form a forming cake;using said yarn of continuous glass fibers without further twisting toweave the glass fabric, without having to resort to the process oftwisting of said yarn of continuous glass fibers previously wound onsaid forming cake.
 4. Method according to claim 3, in which said fabricis comprised of said warp and said weft, whose glass filaments arecharacterized by said flat cross-section, and which fabric is moreovercharacterized by being composed of said continuous glass fibers that donot need the said process of further twisting.
 5. Method for thepreparation of a prepreg, characterized in that it is formed with aglass fabric obtained according to one of the methods cited in claims 1to 4, and brought to a semi-rigid state through impregnation in athermosetting resin bath and subsequent drying.
 6. Glass fabric in whichsaid weft and said warp contain multiple glass filaments and in whichthe filaments of at least one of the two components, weft or warp, hasnot been twisted, and in which the cross-section of the glass filamentis moreover characterized by being flat.
 7. Glass fabric according toclaim 6, characterized in that while said warp yarn has undergonetwisting and the cross-section of said glass filament is roughly round,said weft yarn has not undergone twisting and said filament has a flatcross-section.
 8. Prepreg characterized in that it is obtained by theimpregnation of the glass fabric described in claims 6 or 7 in athermosetting resin bath.
 9. Printed circuit characterized in that itcontains as base material the glass fabric mentioned in claims 6 or 7.